Transference of information via propagating light is generally well understood. For example, a conventional optical fiber communications system may include several III-V semiconductor based optical devices interconnected by one or more optical fibers acting as a transmission medium. Information for transmission may be encoded using well understood techniques. This encoded information may typically be used to modulate a driving signal for an optical source or transmitter, such as a semiconductor laser or Light Emitting Diode (LED). The optical source is responsive to a driving signal to generate a transmission that propagates via the transmission medium to a receiver, such as an optical detector. The optical detector may then provide a signal responsively to the detected transmission to a decoder or demodulator. The demodulator, in response to the received signal, provides the information in a suitable form using well understood techniques. Such methods and systems are well understood by those possessing an ordinary skill in the pertinent arts.
In such systems, control of the transmitter may be established. One type of control is conventionally referred to as wavelength control or wavelength locking. For example, in the case of Wavelength Division Multiplexing (WDM) available channel space may be inversely related to channel spacing, the number of desired channels and data rate, for example. That is, as the desired number of channels increases, available channel-width may generally decrease. Thus, wavelength stability, e.g. locking, in optical communications systems is generally important so as to better ensure that adjacent channels do not unintentionally interfere with one-another. Other reasons for controlling and/or monitoring the wavelength of emitted transmissions are also well understood.
This generally results in strict performance guidelines for optical sources or transmitters, such as lasers, Light Emitting Diodes (LEDs) and Super Light Emitting Diodes (SLEDs). However, such transmitters in operation may tend not to consistently output transmissions of a desired wavelength precisely enough, due to a number of factors including operating temperature and bias current fluctuations, for example.
Laser wavelength control systems, and drawbacks associated with them, are generally discussed in U.S. Pat. No. 5,706,301, entitled LASER WAVELENGTH CONTROL SYSTEM, the entire disclosure of which is hereby incorporated by reference herein.
Some approaches that have been suggested for performing wavelength locking include use of etalons, Fiber Bragg Gratings (FBGs) and dielectric thin film filters. Some drawbacks do accompany these solutions however.
For example, a drawback of an etalon lies in temperature sensitivity of the etalon cavity index, as well as the cavity length, which both may determine the phase of output light. Further, to make the fringe narrow in order to gain resolution, the etalon may need to have a high gain or Q, which typically requires two medium/high reflectivity dielectric mirrors. Costs associated with making such an etalon may be high, due to inclusion of a precision cavity length and high quality mirrors, for example. Further, the FSR (free-spectral-range) may present another concern when using etalons for wavelength locking, due to the periodic nature of their performance in the frequency domain as is well understood by those possessing an ordinary skill in the pertinent arts.
While Fiber Bragg Grating (FBG) filters may be less sensitive to temperature, often the cost associated with the FBG is also high. Further, FBGs tend not to be compact devices and also typically need alignment, which further adds to packaging cost.
Dielectric thin film filters are also generally undesirably temperature sensitive and cost prohibitive. For example, to make a narrow filter many stacked layers of dielectric films may be necessary.
Accordingly, it is highly desirable to provide a method and system for providing cost efficient and relatively temperature insensitive wavelength locking for an optical transmitter, such as a laser.